Steam sent from a boiler or other upstream device to a steam turbine contains solid particles and a phenomenon has long been known in which the solid particles in steam erode components of turbine paths. The solid particles causing the erosion are said to originate in a boiler, a reheater, or their piping. In general, the erosion is particularly noticeable in a forward stage of high-pressure and medium-pressure turbines. The erosion may nonetheless extend to a rearward stage of the turbine depending on the size and quantity of the solid particles.
FIG. 7 illustrates a conventionally typical sealing structure in a steam turbine. In FIG. 7, a nozzle 2 allows steam to flow into a rotor blade 1 and the steam rotates the rotor blade 1. A nozzle outer ring 3 constitutes a nozzle diaphragm that is a structural member with which the nozzle 2 is to be mounted on a casing of the steam turbine.
A plurality of nozzle outer ring sealing fins 4 is mounted through, for example, caulking on an inner peripheral surface of the nozzle outer ring 3. The nozzle outer ring sealing fins 4 block steam that may leak through a clearance between a leading end of the rotor blade 1 and the inner peripheral surface of the nozzle outer ring 3.
In FIG. 7, arrows 30 indicate behavior of solid particles 20 that flow in with the steam. A steam flow that goes through the nozzle 2 has a swirl component and thus tends to be deflected to the outer peripheral side. The solid particles 20 that move with such a steam flow also have a swirl component and, moreover, receive a centrifugal force to be directed toward the outer peripheral direction. As illustrated in FIG. 7, the solid particles 20 deflected toward to the outer peripheral direction collide with the inner peripheral surface of the nozzle outer ring 3; in addition, part of the solid particles 20 enters into the clearance between the nozzle outer ring sealing fins 4 and a rotor blade cover section 5.
A material having hardness lower than that of a body of the rotor blade 1 is generally used for the nozzle outer ring sealing fins 4 in order to reduce adverse effects, such as wear, due to their contact with the rotor blade 1. The nozzle outer ring sealing fins 4 are thus more susceptible to erosion by the solid particles 20. When such erosion develops, the gap between the nozzle outer ring sealing fins 4 and the rotor blade cover section 5 is widened. In addition, the caulking member that fixes the nozzle outer ring sealing fins 4 may be eroded, resulting eventually in the nozzle outer ring sealing fins 4 coming off position. Such erosion may reach a rearward stage beyond an inlet stage of a high-pressure/medium-pressure turbine.
A known arrangement for preventing erosion of steam turbine components, such as the nozzle outer ring sealing fins 4, by the solid particles 20 includes, for example, a circumferential collecting path disposed between adjacent turbine stages. The collecting path can remove the solid particles from the steam.